Semiconductor sensors for detecting biochemical reactions are known in the art, as exemplified by U.S. Pat. No. 4,180,771 to Guckel (1979). FIG. 1 depicts a typical such prior art sensor 2 used to measure the attachment to a solid substrate surface 4 of a desired chemical compound 6 in a solution 8. Sensor 2 typically is fabricated like a metal oxide silicon ("MOS") field effect transistor, wherein region 10 functions like a channel between source and drain regions 12, 14, and region 16 functions like a gate, but without metalization. Using receptor-type mechanisms 18, region 4 is made sensitive to (and encourages adhesion or attraction with) a desired target substance 6. Alternatively, receptor-like mechanisms 18 may be attached to the device gate 16.
Although region 4 should be relatively insensitive and non-reactive to other chemicals, such as 20, in practice region 4 can respond non-specifically and attach with other than desired target substance 6. For example, solution 8 may also include charged particles 22 of varying size, including positive and negative ions. In biochemical sensing applications, a suitable biochemical environment for the receptor 18 and bio-target 6 dictates that test solution 8 have a relatively large ionic concentration. Unfortunately, relatively large ion concentration for the test solution 8 can adversely affect biochemical measurement due to ionic shielding.
Prior art measurements use a reference electrode to obtain stable and reproducible measurements, which electrode may be attached to sensor 2, e.g., electrode 24, or not attached, e.g., electrode 24' (shown in phantom). The reference electrode is coupled to a reference potential V.sub.r, (e.g., ground) and completes an electrical circuit, apparently to provide proper sensor biasing and to eliminate drift. Various bias potentials V.sub.sr, V.sub.gr and V.sub.dr are coupled to the sensor, typically referenced to V.sub.r. One or more measuring devices, indicated generically by 26, are also coupled to the sensor 2.
If the target substance 6 is present in solution 8, it should attach or bind to receptor 18, bringing electrical charges associated with the target substance. Target 6 attachment also brings mass to receptor 18, and can alter receptor 18's contact potential as well.
Thus, during binding or attachment, these electrical charges associated with receptor 18 influence charge present at region 4 (or gate 16, alternatively) and can measurably alter device 2's substrate bias, which can affect device 2's operating characteristics, including conductance and threshold voltage. By monitoring sensor 2 with detection and measurement equipment 26, these characteristic changes may be detected, indicating a binding of the target substance 6.
Further, charges at region 4 can also manifest a potential that tends to vary somewhat logarithmically with the charge concentration, a phenomenon sometimes used in sensing pH. It is characteristic of the prior art that measurements are made when binding of the target substance occurs, e.g., while sensor 2 is still immersed in solution 8.
Unfortunately such prior art sensors and sensing techniques have several deficiencies, including the use of reference electrodes, the inability to meaningfully directly measure charged particles including biochemicals (especially where the test solution is rich in ions), relative device insensitivity and drift, relatively high sensor production cost, and the perceived necessity to make "wet" measurements, i.e., while the sensor is in solution.
Prior art device reference electrode 24 or 24' unfortunately can contaminate the solution 8, and corrupt measurements. Further, the reference electrode bias V.sub.r can interact unfavorably with any ions 22, 30 present in the solution, e.g., resulting in ionic charge separation and polarization. Because even minute movement or agitation of solution 8 circulates these ions, potential disturbances are created that can affect measurement accuracy and introduce drift.
Further, sensing devices and procedures such as depicted in FIG. 1 do not provide meaningful detection and direct measurement of charged particles, especially such particles exceeding a few angstrom in size, where the test solution has high ion concentration. In some applications, the target to be detected is a charged particle 28 that may be several tens of angstroms or greater in size. It is understood that at other than the iso-electronic pH ("pHiso") level, target substances may exhibit a charge of either polarity, depending upon whether pHiso&gt;pH or pHiso&lt;pH, where pH is the test solution pH.
Unfortunately in FIG. 1, ions 22, 30 in solution 8 can screen out and thus mask or shield the target charged particles. Thus, charges associated with the receptors and/or targets may be neutralized (in whole or part), thus masking the desired attachment signal.
Generally, the effects of an electric field operating over a distance upon charges in a semiconductor (e.g., device 2) are understood and used in field effect devices, such as capacitors, field effect transistors ("FETs"), including metal-insulator-semiconductors field effect transistors ("MISFETs"), metal semiconductor field effect transistors ("MESFETs"), and junction field effect transistors ("JFETs").
To better appreciate the adverse effects of ionic shielding, assume that receptor 18 in FIG. 1 has been charged positively (e.g., as a result of pH buffering of the solution 8), and that target material 6 is not yet introduced into the solution. Since solution 8 may includes ions 28, 30 of either polarity, mobile negative ions (assume 30) are attracted to receptor 18, and mobile positive ions (assume 28) are repelled. The polarized negative ions 30 shield or nullify the receptor 18 charge, causing a net charge of zero to be seen somewhat below the substrate surface 4. At the interface between the receptors 18 and substrate-surface 4 the electric field is substantially zero, and thus the underlying FET is not influenced.
When added to the solution, target material 6 binds selectively to the mating receptor 18. But any material 6 charge experiences shielding due to ions in the solution, and produces an indication of net zero, or reduced charge as indicated by the associated insulator electric field as observed somewhat below the substrate surface 4.
Thus, although a charged target material 6 has bound to the receptor 18, shielding prevents meaningful detection by device 2. Device 2's failure to sense attachment is chronic problem with prior art devices, and may result in a false negative report. But to support certain medical and biochemical reactions of interest (e.g., many anti-body-antigen reactions), the solution must have a relatively high ionic concentration that can result in a shielding length substantially masking, reducing or interfering with detection of the binding-charge induced signal of interest.
This apparent resultant low sensitivity associated with prior art FET type sensors (e.g., sensor 2) has caused such devices to be disfavored as sensors for the direct detection of charged molecules in ionic solutions, especially biochemicals.
The prior art has attempted, largely without success, to improve device sensitivity by attempting to make transient measurements, wherein the attachment signal is measured in the brief interval before nullification results from shielding and equilibrium.
Prior art sensor insensitivity is especially troublesome where relatively small changes (.DELTA.S) in a signal (S) are to be measured. Rather than being able to provide a direct measurement of .DELTA.S, such prior art devices sense log(S.DELTA.S) and provide a signal proportional to log(S.DELTA.S)-log(S), at best a relatively insensitive indirect measurement of .DELTA.S. Logarithmic dependent measurements are believed to account for the relatively low sensitivity of typical prior art pH sensors.
Ionic shielding is not the only disadvantage with prior art in-solution sensor measurements. Wet testing can subject the measurements to drift resulting, for example, from ion movement within the solution, and from reference electrode contamination.
Further, in a given application the measurement and detection equipment 26 may require sophisticated and expensive components. Under such circumstance, having to "wet test" requires that the test and detection/measurements occur essentially at the same time and place as the target binding. This restriction can preclude the use of sensors if sophisticated equipment is not readily available in the region where the testing (that possibly leads to binding) occurs.
It would be advantageous if after possible binding, the sensor could be sent, preferably dry, to a remote facility for detection and measurement of any target substance attachment using sophisticated equipment not available at the testing/binding region. Unfortunately, such "dry testing" is not practiced with prior art devices and procedures such as depicted in FIG. 1.
Fabricating many prior art sensor devices is sufficiently expensive as to preclude "use once and discard" practice. Similarly, often the receptor material is scarce or very expensive. Clearly it would be advantageous if devices and/or their receptor materials could be used more than once. In addition, sufficiently inexpensively fabricated devices could be provided in arrays to permit simultaneous testing for multiple target materials, e.g., multiple disease antigens.
Many prior art sensors have limited sensitivity, limited sensor gain, and/or device drift, unfortunate limitations since in many clinical applications, a target biological analyte may exist in a minute concentration, i.e., a few ng/ml for proteins in blood serum. Substantially more sensitive devices would permit the simultaneous use of several different dedicated receptors to provide more rapid (and thus less expensive) testing, including differential analysis testing.
In short, there is a need for an inexpensive field effect type biosensor, preferably a IC-compatible (thus permitting integration with signal enhancing, control and other environmental sensors, all on-chip), that can be inexpensively mass produced using standard semiconductor fabrication technology. Such device should reliably measure biochemical information with high sensitivity, and be substantially free of signal drift.
Further such devices should include multiple receptors, some of which may be dedicated to binding different target materials, and should further include a mechanism for discerning which of several target materials have in fact bound. Further, there is a need for devices that may be fabricated and used in arrays, including arrays containing sensors with multiple types of receptors. Such arrays can promote rapid and relatively inexpensive testing, including differential and confirmational analysis testing. Further, arrays can provide self-testing of the devices themselves, as well as confirming the presence of a suitable environment for valid testing, e.g., acceptable ranges of temperature, and pH.
Preferably such device, and a methodology using such device, should not require a reference electrode, and should be capable of making measurements under wet or dry conditions. Further such device and method should enable detection of a contact potential resulting from the binding of a target material and a receptor, and should include mechanisms to eliminate false positive and false negative measurements.
Further, such device and methodology should provide mechanisms for enhancing the sensitivity of the device per se, for enhancing the effective amount of charge binding to the device, and for amplifying the signal detected by the device. Preferably such mechanisms should be usable and reusable under wet or dry measurement conditions. Preferably measurements could be made not on a transient basis, but by integrating all charge captured by the sensor receptors over preselected time period. Such technique would provide an enhanced signal, enhanced signal-to-noise, and would lend itself to differential analysis.
Finally, such device and methodology should be useful in a wide spectrum of applications including biochemical sensing and measurement, DNA research, pH and hydrogen sensing, pollution sensing, optical and photodetector sensing, pyroelectric sensing, magnetic and force sensing including piezoelectric sensing.
The present invention provides such devices and methodologies.